The present invention relates to furnaces and particularly to apparatus for automatically cleaning ports introducing combustion air into the firebox of a chemical recovery furnace.
Wood pulp for papermaking is usually manufactured according to the sulfate process wherein wood chips are treated with a cooking liquor including sodium sulfide and sodium hydroxide. The wood chips and the cooking liquor, called "white liquor," are cooked in a digester under predetermined heat and temperature conditions. After cooking, the used liquor, termed "black liquor," containing spent cooking chemicals and soluble residue from the cook, is washed out of the pulp and treated in a recovery unit where the cooking chemicals are reclaimed. Without reclamation and reuse of the cooking chemicals, the cost of the papermaking process would be prohibitive.
In the recovery process, the black liquor is first concentrated by evaporation to a water solution containing about 65 percent solids, which solution is then sprayed into the firebox of a black liquor recovery boiler, a type of chemical reduction furnace. The chemical reduction furnace is a reactor wherein the processes of evaporation, gasification, pyrolysis, oxidation and reduction all occur interdependently during recovery of the cooking chemicals. The organic materials in the black liquor, lignin and other wood extracts, maintain combustion in the firebox, and the heat produced melts the spent cooking chemicals. A molten smelt flows out of the furnace through a smelt spout to a collection tank. Concurrently, combustion heat is employed to generate steam in a wall of boiler tubes for use as process steam and for generating electricity.
The combustion process requires the introduction of large volumes of air into the firebox, air comprising about 80 percent of the material entering the furnace. The air is forced into the firebox from wind boxes or ducts disposed at several levels in surrounding relation to the firebox, through a plurality of air ports in the walls of the furnace, viz.: primary, secondary, and tertiary air ports.
The black liquor sprayed into the firebox, having a consistency similar to that of warm 60 weight oil, swirls, burns and falls toward the bottom of the firebox as combustion products comprising char material and smelt. The smelt and char material contact the outer walls of the firebox and, cooled by the inflowing air, form excrescent deposits around edges of the air ports, particularly along the edges of the openings where the excrescent material builds up under influence of air rushing through the air port. Such buildups of char material can block air flow through the ports by as much as ten percent, and can even block individual ports completely. In accordance with prior practice, the char build-up is periodically removed by a worker inserting a rod into the air ports successively around the boiler. With manual rodding of the air ports, gradual build-up of char material intermittently around the furnace still causes changes in the volume of combustion air, as well as changes in air distribution, velocity and pressure. Therefore, furnace operation tends to be inefficient and unpredictable with an attendant decrease in the quantity of chemical that can be recovered, a decrease in the amount of steam produced per unit of fuel, and increased emission of noxious gases such as carbon monoxide and sulfur dioxide.
Apparatus for automatically cleaning openings in a recovery furnace is known--see U.S. Pat. No. 4,748,004, entitled APPARATUS FOR CLEANING AIR PORTS OF A CHEMICAL RECOVERY FURNACE, and U.S. Pat. No. 4,423,533, entitled FURNACE AIR PORT CLEANER. The apparatus disclosed in these patents includes rods with cleaning tips attached to ends thereof, and the cleaning action comprises a wiping motion accomplished by inserting the cleaning tips into corresponding openings and then changing the position of the rods, while the cleaning tips are within the openings, so as to cause the cleaning tips to move lengthwise of the openings to dislodge the build-up therein. It has been found that particularly hard deposits of solidified smelt and char material resembling a vitreous substance may resist the wiping action of the cleaning tip and in extreme instances may cause individual rods to stress and even bend. It is desirable therefore to provide a cleaning action in which the cleaning tip acts as a ram, i.e. wherein force is applied longitudinally of the rod while still being able to clean the longitudinal height of the air port opening without disrupting or blocking combustion air flow.
Apparatus in accordance with U.S. Pat. No. 4,822,428 entitled APPARATUS FOR CLEANING AIR PORTS OF A CHEMICAL RECOVERY FURNACE comprises a plurality of rods each having a cleaning tip adapted for insertion through a furnace air port, as a result of longitudinal translation of the cleaning rod, for dislodging excrescent material. The cleaning tip is then retracted and indexed to a different location along the narrow opening, and the insertion operation is repeated. Excrescent material is forceably removed along the entire air port without wiping the cleaning tip along the opening when it is inserted in the air port. The described apparatus has proved quite advantageous especially in the case of primary air ports near the bottom of a furnace close to the char bed. These air ports are relatively long and narrow and the cleaning tip has a width comparable to the narrow dimension of the air port. The tip is indexed vertically, i.e. along the direction of the greater dimension of the air port, for repeatedly ramming through the deposited material.
Secondary and tertiary air ports, on the other hand, are higher up in the furnace wall and have larger width dimensions. Also, the width dimension typically varies as dictated by the outline of boiler tubes inside the furnace wall which are spread to accommodate the air port. Neither the aforementioned air port cleaner having a wiping movement within the air port opening, nor the air port cleaner which successively rams through the deposited material along a narrow air port are advantageous in the case of secondary and tertiary air port cleaning.
Furthermore, although an air port cleaner consisting of a large plate or ram may be utilized for cleaning larger air ports, the use of these larger dimensioned devices is disadvantageous in that they may block the flow of air through the port when the cleaning operation is taking place. It would be desirable to be able to employ an air port cleaner having a cleaning tip which is relatively small in comparison to the opening being cleaned.